The present invention relates to the art of calendering in general, and more particularly to improvements in a method of and an apparatus for calendering webs of synthetic plastic material. Still more particularly, the invention relates to improvements in a method of and an apparatus for calendering tapes of synthetic plastic material one side of which is coated with a layer or film of magnetic material. Typical examples of such webs are magnetic audio and video tapes.
It is already known to treat magnetic audio and video tapes in a calender wherein successive increments of the tape are caused to pass through several nips which are defined by pairs of calendering rolls and wherein the tape is heated in the course of the actual calendering operation, i.e., during passage through the nips. As a rule, a magnetic audio or video tape contains a very thin web of synthetic plastic material (the thickness is normally in the range of less than 10 .mu.m) one side of which is coated with pulverulent magnetic material in a suitable binder. The calendering operation is intended to bring about an equalization of the thickness of the applied magnetic film as well as to enhance the smoothness of the exposed side of the film. Heretofore known calenders which are used for the treatment of magnetic tapes comprise a stack of superimposed calender rolls and a set of guide rollers which are adjacent to the nips between neighboring calendering rolls of the stack and direct successive increments of the running tape into the adjacent nips. At least some of the calender rolls have rims which consist of or contain a viscoelastic natural or synthetic material. The rims of the remaining calender rolls are hard; they are made of steel or constitute hard casting made of a suitable metallic material. Some of the calender rolls are heated, for example, by causing a gaseous or liquid heat exchange medium to flow therethrough. In other words, the calender is designed in such a way that it heats spaced-apart portions of the running magnetic tape.
The just discussed calenders have been found to ensure a reasonably satisfactory treatment of magnetic tapes. However, the quality of the thus treated tapes is not uniform and it cannot be increased beyond a rather low average quality. In other words, there are limits as to the maximum achievable quality of magnetic tapes which are treated in accordance with the above outlined method and in the above described calenders.
German Auslegeschrift No. 11 13 357 discloses a calender wherein a primary calender roll having a soft elastomeric rim is surrounded by a set of four secondary or satellite calender rolls each of which has a hard metallic rim and each of which defines a nip with the adjacent portion of the peripheral surface of the primary roll. The German publication further discloses the possibility of heating the secondary calender rolls. A pair of neighboring secondary rolls is rotatable about fixed axes and the other two secondary rolls can be moved toward and away from the axis of the primary roll which floats between the four secondary rolls and bears against the secondary rolls of the pair with a force which is determined by the other two secondary rolls. The just described calender is designed primarily for the treatment of paper webs but mention is also made of the possibility of treating webs of synthetic material, webs of textile material or coated webs of flexible material without specifically mentioning magnetic audio and/or video tapes.
German Offenlegungsschrift No. 2 405 222 discloses a method and apparatus for treating magnetic video tapes by causing them to pass through the nip of a metallic roll and a second roll having an elastic rim made of polyurethane. Each of the rolls is movable at right angles to its axis and the metallic roll is heated so that the tape is heated during passage through the nip. The Offenlegungsschrift proposes to cool the second roll so as to prevent the temperature of its peripheral surface from rising above 100.degree. C. The Shore hardness of the rim of the second roll is supposed to be between 75.degree. and 85.degree., and the pressure in the nip should be in the range of between 150 and 300 kg/cm.sup.2.